The invention relates to improvements in the solution polymerization of low molecular weight cyclic dichlorophosphazene oligomers to higher molecular weight substantially linear polydichlorophosphazene polymers. More particularly, the invention relates to the use of cycloaliphatic solvents and certain boron catalysts in the solution polymerization of substantially pure cyclic dichlorophosphazene oligomers to form substantially linear, substantially gel free, polydichlorophosphazene polymers.
The polymerization of low molecular weight cyclic dihalophosphazenes such as (NPCl.sub.2).sub.3 or (NPCL.sub.2).sub.4 to higher molecular weight linear polydihalophosphazene polymers is well known in the art.
Prior processes for polymerizing such cyclic dihalophosphazenes involved the uncatalyzed or catalyzed thermal polymerization of such cyclic dihalophosphazenes principally employing a bulk process without solvents. Such prior bulk processes are described in U.S. Pat. No. 3,370,020 to Allcock, et al. issued Feb. 20, 1968; U.S. Pat. No. 3,515,688 to Rose issued June 2, 1970; U.S. Pat. No. 4,005,171 to Reynard, et al. issued Jan. 25, 1977; U.S. Pat. No. 4,123,503 to Snyder, et al. issued Oct. 31, 1978 and in the publication entitled "Phosphorus-Nitrogen Compounds" by H. R. Allcock published in Academic Press, New York, N.Y. 1972.
While such prior bulk polymerization processes provided for the conversion of cyclic dihalophosphazenes to linear high molecular weight polydihalophosphazene polymers, they exhibited several significant disadvantages. Thus, for example, the uncatalyzed bulk polymerization process required relatively high polymerization temperatures, e.g., 200.degree. to 300.degree. C. and lengthy polymerization times (i.e., slow conversion rates). As will be apparent, such high temperatures and lengthy polymerization times require significant amounts of energy. Moreover, these prior bulk polymerization processes often resulted in the formation of gelled or partially gelled polydichlorophosphazene polymers which were insoluble in many conventional solvents and therefore could not readily be derivatized to produce useful polymeric products such as those described in, for example, U.S. Pat. Nos. 3,370,020; 3,515,688; 3,700,629 and 3,856,712.
In addition, bulk processes for preparing polydihalophosphazene polymers lead to difficult and often complex problems in transferring the molten polymer from the polymerization vessel to a reactor for subsequent derivatization procedures of the type described in the aforementioned patents.
In view of the foregoing problems associated with bulk polymerization processes, those active in the polyphosphazene polymer field have increasingly turned their attention to solution polymerization processes. One such solution polymerization process is described in U.S. Pat. No. 4,005,171 to Reynard, et al. issued Jan. 25, 1977. As described in that patent, cyclic dihalophosphazenes are solution polymerized in nitro or halo aromatic solvents such as nitrobenzene, chlorobenzene, 1,2-dichlorobenzene, 1,2,4-trichlorobenzene, o- and m-nitrotoluene and the like. While the solution polymerization process described in the patent is advantageous in several respects, it also suffers significant disadvantages. Thus, the aromatic solvents employed in the process are not sufficiently inert and many react with the polydichlorophosphazene polymers at higher temperatures. In addition, the removal of aromatic solvents from the finished polymer and recovery of such solvents for reuse presents further problems.
Another approach to such solution polymerization processes is described in copending application Ser. No. 739,212 of Halasa, et al. filed Nov. 5, 1976 now U.S. Pat. No. 4,225,567, commonly assigned to applicants' assignee herein. As described in that application, the process involves the solution polymerization of cyclic chlorophosphazenes in aromatic hydrocarbon or cycloaliphatic hydrocarbon (including cyclohexane) solvents in the presence of small amounts of modifiers selected from sulfur, sulfur compounds or sulfur donating compounds.
While the process disclosed in copending application Ser. No. 739,212 is advantageous in many respects and permits the solution polymerization of cyclic chlorophosphazenes to higher molecular weight linear polydichlorophosphazene polymer without the formation of appreciable amounts of gel, it also suffers from certain disadvantages. Thus, as disclosed in the copending application, it is necessary to employ a sulfur modifier along with the aromatic hydrocarbon or cycloaliphatic hydrocarbon solvent in order to prevent the formation of gelled polymer. However, the use of the sulfur modifier leads to other problems. Hence, the use of a sulfur modifier can lead to the possibility of sulfur being incorporated into the polyphosphazene polymer backbone which in turn can adversely affect the thermal stability of the finished polymer.